# NTSB SEA94LA153: Schweizer 269C, N7506A, Hillsboro, OR, June 21, 1994 - Type: NTSB accident investigation - Classification: Accident · No injuries · Limited investigation · Final report - Date: June 20, 1994, 18:00 local (Jun 21, 1994, 01:00 UTC) - Place: Hillsboro, OR - Coordinates: 45.439018, -122.959533 (estimated) - Nearby events: https://openairdocs.org/map.md?lat=45.4390&lon=-122.9595&mi=10 - Phase of flight: [Landing](https://openairdocs.org/ntsb-phases/550) (OpenAirDocs' grouping) - Highest injury: No injuries - Injuries: 2 uninjured - Status: Completed, report January 12, 1995 - Report: Approved by Office Director - NTSB case key (mKey): 41956 - NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=SEA94LA153 - NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/41956/pdf - Page: https://openairdocs.org/ntsb/SEA94LA153 ## Probable cause THE IMPROPER SERVICING OF THE SKID SHOCK STRUTS. ## Sequence of events In NTSB's coding before 2008 (https://www.ntsb.gov/GILS/Documents/codman.pdf). - Loss of control - on ground/water, landing - flare/touchdown - landing gear,main gear shock absorbing strut - pressure too low (cause) - maintenance,service of aircraft/equipment - improper (cause) ## Analysis WHILE EXECUTING A RUN-ON LANDING THE INSTRUCTOR FELT A 'THUMP' AND THE HELICOPTER BEGAN TO OSCILLATE LATERALLY. THE INSTRUCTOR PILOT DIAGNOSED THE PROBLEM AS GROUND RESONANCE, TOOK CONTROL OF THE AIRCRAFT FROM THE STUDENT, CLOSED THE THROTTLE, LOWERED COLLECTIVE, AND THE HELICOPTER DEPARTED THE TAXIWAY. POST CRASH EXAMINATION OF THE SKID SHOCK STRUTS REVEALED THAT ALL FOUR WERE CHARGED AT A PRESSURE LOWER THAN THAT SPECIFIED. THE MAINTENANCE HANDBOOK WARNS THAT IMPROPER SHOCK STRUT PRESSURE LEVELS MAY RESULT IN GROUND RESONANCE AND DESTRUCTION OF THE HELICOPTER. ## Factual narrative On June 20, 1994, approximately 1800 hours Pacific daylight time (pdt), a Schweizer 269C helicopter, N7506A, registered to C&H Helicopters, Inc., operated by Hillsboro Helicopters, Inc., and under the control of Neal H. Lawson, a commercially certificated helicopter instructor pilot, was substantially damaged during an encounter with ground resonance while the student pilot was executing a run-on landing to a parallel taxiway at the Hillsboro Airport, Hillsboro, Oregon. The pilot in command and Peruvian student pilot were uninjured. Visual meteorological conditions existed at the time and no flight plan had been filed. The flight, which was instructional in nature, was to have been operated in accordance with 14CFR91 and originated at Hillsboro at 1745 hours. The pilot in command reported that during the student's run-on landing he "felt a "thump" in the airframe and the aircraft began to oscillate laterally." The pilot took control of the aircraft diagnosing the problem as ground resonance and closed the throttle and lowered collective. The aircraft began to rotate to the right and eventually came to rest at the edge of the taxiway (refer to photographs 01 and 02). The four landing gear (skid) shock struts were removed from the aircraft and tested at Schweizer Aircraft Corp., under the control of Donald F. Miller, FAA Principal Maintenance Inspector (refer to attached Inspector Statement). The load stroke tests revealed that all four of the shock struts exhibited a charge pressure lower than that specified (refer to pages 1/2 of attached Report No. SA269C-30-7). Additionally, revision No. R-19 from the Handbook of Maintenance Instructions (HMI) contains a warning stating in part that improper pressure may result in ground resonance and destruction of the helicopter (refer to Attachment R-19). ## Aircraft - Registration: [N7506A](https://openairdocs.org/deregistered/N7506A-2002-09-17), registration cancelled 2002-09-17, S/N S1280, built 1987 (OpenAirDocs) - Reported make and model: SCHWEIZER 269C - Registry model: [Schweizer 269C](https://openairdocs.org/model/schweizer-269c) (registry code 8059502) (OpenAirDocs) - Registry engine: [Lycoming HIO-360 series](https://openairdocs.org/engine/lycoming-hio-360-series) (registry code 41524, reciprocating) (OpenAirDocs) - Category: Helicopter - Serial number: S1280 - Airframe hours: 664 - Engines: LYCOMING HIO-360-D1A 190 horsepower reciprocating - Damage: Substantial damage - Flight: Instructional, Part 91 - Operator: Hillsboro Helicopters, Inc. - Registered owner: C & H Helicopters, Inc. - Airworthiness certificate: Standard, Normal - Last inspection: Annual, June 17, 1994, 5 hours before - Airport: HIO, Portland-Hillsboro ## Flight crew As NTSB recorded them. NTSB leaves out names. - Pilot; age 36; commercial; airplane ratings: single-engine land; medical: class 2, valid medical--w/ waivers/lim; seat: right; flight hours: 605 total, 13 in this make and model, 525 as pilot in command, 129 in the last 90 days, 44 in the last 30 days, 2 in the last 24 hours ## Weather - Conditions: Visual (VMC), Day - Wind: 20° at 9 knots - Visibility: 50 statute miles - Lowest ceiling: none - Temperature: 82 °F, dew point 54 °F - Altimeter: 29.00 inHg - Observed at: HIO ## FAA reports on this event (1) The FAA's own accident and incident reports, matched to the aircraft by N-number and date (OpenAirDocs). - [Accident 19940620015739A](https://openairdocs.org/incident/19940620015739A), Jun 20, 1994: Hughes 269C, Portland, OR, Substantial damage. Thump on landing. CFI diagnosed ground resonance. Took over too late. Low pressure shock struts. Warning in AFM. From the NTSB's public API. Aircraft are matched to registry aircraft by N-number and event date, checked against the reported serial number, make and model; the registry's history has gaps, so older matches can be wrong. Compare the reported make and model. --- OpenAirDocs.org is an independent research project of [Contrail Labs](https://contraillabs.com/). It is not affiliated with the FAA or the NTSB. Source documents are U.S. government works in the public domain.